During the Korean War, Colonel  Henry Blake commands the 4077th Mobile Army Surgical Hospital (MASH), only miles from the front line.  A lax military leader, the married Blake is more concerned with his lover and running the hospital than following any military protocol.  Meanwhile, his right-hand man, Corp.  “Radar” O’Reilly, who has an uncanny ability to recite Blake’s every command before he can utter it, manages the necessary bureaucratic red tape.

In crowded and bloody operating tents, the short-handed staff, equipped with rapidly diminishing supplies, deals with dozens of wounded soldiers in twelve-hour shifts.  Even the sincere yet ineffective Father John Mulcahey, also known as “Dago Red,” must stop reading the last rights to a dying man to assist in surgery.  Upon Blake’s request for additional help, surgeons Duke Forrest and Hawkeye Pierce are sent to the 4077th.  While extremely competent, the recently drafted Duke and Hawkeye lack any respect for military decorum.  To maintain their sanity amidst the constant flow of death and mayhem, they flirt with the nurses, arrogantly quip in surgery and play practical jokes on their roommate, the fanatically pious and taciturn Major Frank Burns.

After watching Burns teach Korean mess hall boy Ho-jon to learn English by reading the Bible out loud, Duke and Hawkeye decide the sixteen-year-old would have more fun practicing with Playboy and then teach him how to make martinis as their cabin boy.  Fed up with Burns’ pious and humorless behavior, Duke and Hawkeye demand that Blake remove him from their tent.  Pressured by the impending arrival of more wounded, Blake agrees to remove Burns and to get a “chest cutter,” the doctors’ other stipulation before they concede to operating.

Days later, Hawkeye and Duke welcome thoracic surgeon and new roommate Trapper John McIntyre to The Swamp, their newly renamed tent.  Trapper easily wins the men’s affection by providing hard-to-get olives for their martinis, but coyly eludes their questions about his past.  Days later, Hawkeye finally recognizes Trapper as a former college football star when Trapper expertly catches a football pass, and also realizes Trapper is a preeminent surgeon.  The two then become fast friends.

One day at the hospital, Trapper watches as Burns, covering for his own malpractice, blames a patient’s death on Private Boone, who is stricken with despair over the incident.  Furious about the irreparable harm Burns inflicts with his inept work, Trapper punches him just as Blake and the officious new chief nurse Major Margaret Houlihan pass by.  Houlihan is incensed by the lack of decorum and further insulted by Hawkeye’s practice of addressing the staff by their first names.  After she insists to Hawkeye that Burns is an excellent military doctor, he caustically replies that not only is he no longer interested in sleeping with the prudish Houlihan, but thinks she is a “regular Army clown.”

One night as Houlihan and Burns draft a letter to protest Hawkeye and Trapper’s behavior, they are sexually aroused by their mutual respect for military law.  Meanwhile, Radar sets up a microphone in Houlihan’s tent and broadcasts their passionate cries over the camp intercom system until the horrified couple realizes that the entire camp is listening in.  The next morning Duke and the others taunt Houlihan with her new nickname “Hot Lips” and provoke Burns with questions about his sexual acts.  When Burns physically attacks Hawkeye, Blake, believing the fight to be unprovoked, sends Burns away in a straight jacket.

Days later, dental officer Captain  Waldowski, famous for sexual prowess and thus nicknamed “Painless Pole,” admits to Hawkeye that he has experienced one night of impotence.  Believing psychological texts suggesting that his overt heterosexuality is just a cover for latent homosexuality, Painless decides to commit suicide to avoid facing his three fiancées back home.  When Painless asks for assistance, Hawkeye suggests the “black capsule,” a quick end to his life.  Dressed in white lab coats, the surgeons and friends prepare a suicide “last supper” in which they break bread and drink wine with Painless, before he climbs into a coffin to take his pill and die.  That night, Hawkeye convinces the soon-to-be-discharged Lieutenant  Dish, a married nurse with whom he has been having an affair, that she is obliged to have sex with the now-unconscious Painless to restore his “health.” The next morning, Painless wakes fully restored, while Dish leaves for home blissfully satisfied by Painless.

Days later, the surgeons decide to bet on whether Houlihan is a “real” blonde and, needing proof, gather the camp outside the women’s shower and pull up the tent while Houlihan bathes.  Humiliated and enraged, Houlihan demands that Blake fire Hawkeye and the others, threatening to resign her commission, but Blake instead suggests that she resign.

Later, when Ho-jon is forced to have a medical examination to determine his eligibility to serve in the Korean army, Hawkeye gives him medication to cause temporary heart acceleration and low blood pressure to ensure that he is rejected.  Suspecting the ruse, the Korean doctor keeps the boy as Hawkeye watches powerless to stop him.  Soon after, Trapper receives orders to go to Kokura, Japan to tend to a United States congressman’s son and takes Hawkeye with him.

Arriving at the Kokura hospital with their golf clubs, Hawkeye and Trapper demand to start the operation immediately so they can play a round before dark, despite the head nurses’ protests that they must first have commanding officer Col.  Merrill’s approval.  When Merrill barges into the operating without scrubs demanding an explanation, Hawkeye tells him that he will be to blame if the boy dies from infection caused by Merrill’s unsterilized intrusion.

During surgery, anesthesiologist “Me-Lay” Marston, Hawkeye’s old friend, invites them to visit a brothel after surgery, explaining that the establishment doubles as a children’s hospital, where Me-Lay moonlights for surgeries.  While being entertained by the prostitutes, an emergency arises involving a child of an American soldier and Japanese prostitute.  Hawkeye and Trapper take the child to the military hospital, but Merrill refuses to serve “natives.” To prevent any military action against themselves or the child, Melay and the surgeons use the sedation gas on Merrill and take compromising photographs of him with a prostitute to use as blackmail.

Returning to 4077th in their golf attire, complete with knickers and argyle socks, Hawkeye and Trapper go straight into surgery.  Later, when General  Hammond arrives at the camp to investigate Houlihan’s formal complaints about the surgeons, Hawkeye, Duke and Trapper, aware of Hammond’s football obsession, distract him with the suggestion that they stage a football match between Hammond’s 325th and the 4077th, a team that has yet to be created.  Hammond agrees on the condition that Blake place a $5,000 bet on the outcome of the game.  Needing a fail safe team fast, the surgeons tell Blake to request surgeon Oliver Harmon “Spearchucker” Jones, once a star player for the Philadelphia Eagles.

After several weeks of training, the 4077th team plays Hammond.  Hawkeye, realizing that Spearchucker is their only real chance of winning, hides his identity from Hammond and keeps him out of the game until the second half.  During the first half, Blake orders a 4077th player to inject a sedative into the opposing team’s star player, ensuring his removal from the game.  In retaliation for a racial slur from a 325th player, Spearchucker coaches his teammate to insult the player’s sister, which results in a fight that leads to another 325th player being banned from the game, thus ensuring the 4077th’s victory.

Days later back at camp, Hawkeye and Duke receive immediate orders to be relieved of their duty and return home.  Unsure of what welcome awaits them, the men prepare to leave, while Mulcahy blesses their Jeep from his prayer book and the war continues on around them.